Soft errors are usually the results of radiation from alpha particles from semiconductor packages and neutrons from the environment. A soft error changes data (e.g., the states in a storage node), but does not change the physical circuit itself. If the data is rewritten, the circuit works again. Soft error rate (SER) is the failure rate at which a device (or system) encounters or is predicted to encounter soft errors. SER is typically expressed in FIT (failures in time). The unit adopted for quantifying FIT is also called FIT, which refers to 1 error per billion hours of device operation. Logic SER shows an increasing trend with technology scaling.
Radiation hardening refers to making memory elements resistant to damage or malfunctions caused by radiation, e.g., from neutrons and alpha particles, etc. For example, in a flip-flop, radiation hardening enables the flip-flop to experience fewer changes to its state (e.g., flip the data). In advanced semiconductor process technologies, storage nodes, including those of SRAM and flip-flops, are more susceptible to single event upsets (SEUS) from radiation. Error detection and correction (ECC) circuits to improve radiation hardening usually require complex and elaborate implementations, and therefore also require large circuit areas in a semiconductor chip. Additionally, some correction circuits require complicated system feedback and controls (e.g., circuitry to provide correction information through a feedback loop to replace erroneous information).
Various approaches use feedback or redundant circuits for correcting storage nodes due to radiation, but they require circuits (e.g., transistors) being modified and/or added to existing correction circuits and flip-flops, which are also susceptible to SER due to radiation.
Other approaches add gate capacitors to improve the ability to sustain charge injection into the storage nodes by increasing critical charge (e.g., Qc), which helps improving SER FIT. Gate capacitors, however, are also susceptible to radiation because they include active gate regions, which are proportionally related to SER. Additionally, gate capacitors cannot be used across the input and output of the memory elements (e.g., flip-flops) because a gate capacitor requires a power or ground node.
Like reference symbols in the various drawings indicate like elements.